UNIVERSITY OF HAWAII LIBRARY

~NIVrFISITY OF HAW,\ ?! l.l!"::l,",il'{

PRilSM WORKING PAPER

No. 90-003

VSAT DATA NETWORK

NORMAN ABRAMSON

ABSTRACT

VSAT DATA NE'l'WORXS

Norman Abramson Department of Electrical Engineering

University of Hawaii Honolulu, HI 96822

Two general methods have been used to provide random access packet communications in Very Small Aperture (VSAT) data networks -- Spread Spectrum (or CDMA) and ALOHA. In this paper we review the use of satellite channels for such networks and discuss certain basic aspects of the architecture of VSAT data networks. Although Spread Spectrum and ALOHA have different origins and are sometimes r epresent ed as competing technologies, they can in fact be characterized as different ways of viewing the same low dimensional signals in a high dimensional s ignal space . After a brief introduction to the architecture of VSAT networks we show how a simple linear transformation of conventional ALOHA packets leads to signals identical in all r espects to the most common form of Spread Spectrum signals. We call the result of this transformation spread ALOHA.

There are two practical consequences of this theoretical result. First, for the case of small earth stations it is not possible to find an access technique with a higher throughput than Spread ALOHA. Second, the . use of different spreading sequences for different users in a packet network using Spread Spectrum is not necessary for user separation.

l..

VSAT DATA NETWORKS

Table of Contents

INTRODUCTION

1.1 1.2 1.3 1.4

VSAT Architecture Access Techniques History .... ..... Satellite Multiple Access

.. ....

2. NOTATION

2.1 2.2

Packets and Matched Spreading Sequences

3. SPREAD ALOHA

Filters .......

3.1 Spreading the Packets . . . . . . . . . . . . . . . ..... 3.2 B,it Spread ALOHA . . . . . . . . . . .. 3.3 Discussion .. . . . . 3.4 Conclusions . . . . . . . . . . . . .... . . . .

i

Page

l 4 8

10

14 18

. . . . 19 • • • 20

26 27

1. INTRODUCTION

1.1 VSAT Architecture

A communication satellite can be used to provide conventional

point-to-point channels for data networks and other applications.

But the key characteristic of communication satellites for many

applications is the broadcast and multiple access nature of the

satellite channel. For the physics of the satellite path

provides the built in capability of one to many (broadcast)

channels and of many to one (multiple access} channels to and

from any earth station in the satellite footprint. It is this

characteristic of satellite communications which can be used to

provide new types of service which are simply not practical or

not economical using conventional ground communications.

The broadcast nature of the satellite channel encouraged the

development of satellite communications for video and audio

distribution in the l970's. This same broadcasting capability of

satellite channels bas been employed for a variety of one way

data distribution networks since the early 1980's. During the

last few years we have seen the development of a powerful and

flexible network architecture which coll!hines the capabilities of

satellite broadcasting and satellite multiple access for two way

data networks composed of very small aperture terminals (VSAT's).

The network architecture used in VSAT data networks is almost

always designed around a single large (hub) earth station

transmitting data in a broadcast channel to a large number of

VSAT's.

l

-i-i ... -i Hub USAT'a

Figure 1: A VSAT Network

The VSAT's in such a network transmit data in packets to the hub

station using the multiple access capability of the satellite

channel. Since there are no direct links from one VSAT to

another, any VSAT to VSAT traffic must take a two hop path from

its source to its destination. There are two good reasons for

such a limitation. First, transmit power and r eceive power

requirements on the l arge nwnber of VSAT's in such a network can

be relaxed considerably by the fact that the uplink and downlink

in the network each benefit from the higher performance

capabilities of the hub station. Second, many of the

applications which can be nefit from the use of VSAT t echnology

require communications from a large number of users to a central

information resource. These appli cations include large financial

networks, reservation networks and shared data bases.

2

The link from the hub station of a VSAT network to ~he VSAT's is

easily confi gured using conventional time di vision multiplexing

(TOM), And although there are differences in data rates, in

modulation techniques and in transmission formats among different

VSAT networks there i s general agreement on the use of TOM for

multiplexing from the hub to the terminals.

µ

----~ ~ ii · ... i

USAf0

•

Figura 2: Broadcast Channel - Hub to VSAT's

The multiple access link from the VSAT's to the hub however, has

been subjected to a greater degree of variation in data networks

which have been built during the past decade. And in the overall

design of the network the choice of .an access technique for the

VSAT to hub multiple access channel is the key decision which

must be made by the network architect. There are still a nwnber

of important choices in data rates, modulation t echniques and

transmissi~n formats. But while these differences will have a

maJor impact on overall network performance i t i s probably

accurate to characterize the ·choice of access technique as the

primary feature which distinguishes one network from anothe r. In

this paper we focus on the choice of access techniques for VSAT

networks and analyze the architecture of these networks,

3

; ):( .. ;

; ;

' ; ,:.

It

....

Figure 3: Multiple Access Channel - VSAT•s to Rub

1.2 Access Techniques

Frequency Division Multiple Access (FDMA) and Time Division Multiple

Access (TOMA) are the two primary access techniques used for sharing

the high ~pacity of a typical commercial satellite transponder

all!ong several earth stations in a voice network. With either of

these t echniques the total transponder capacity is divided into

frequency or time slices for use in fixed allocation channels of

moderate capacity. When the traffic from a single tet'lllinal in a

network is bursty the efficiency of this kind of operation drops

and a De:mand Assigned Multiple Access (DAMA) t echnique is

required in order to allocate capacity in response to fluctuating

user demands.

In a DAMA system network capacity is allocated (either in

frequency or time slices) in res ponse to user requests for

channel capacity. Note however that in one sense the use of a

DAMA system merely shifts. the problem of allocating capacity in

response to random requests to another l evel. A special channel,

usually called the request channe l, is assigned to carry requests

for capacity assignments in a DAMA network and some access method

4

to allow reasonably efficient sharing of the request channel mus t

be found.

During the past few years it has become practical to build VSAT

networks composed of hundreds or thousands of small (typically

1.2 meters in di!lllleter) t erminals. The traffic in these networks

is usually in the form of single data packets originating from

interactive users or bursts of data packets originating from some

file transfer protocol. In general as the number of earth

stations in a network increases the more the traffic from a

single station will appear to fluctuate due to random user

demand. In such a network of course the use of fixed assignment

FOMA or TOMA is usually impractical, while the use of DAMA would

ordinarily impose an unreasonable amount of overhead in the

network. In order ~o provide multiple access to packet data

networks, therefore two other access techniques have been used

--- Spread Spectrum Multiple Access (or Code Division Multiple

Access · - COMA) (l] and ALOHA (2) (in a so called unslotted or

slotted mode).

Spread spectrum has been defined as communications which uses a

"bandwidth well beyond what is required to transmit digital data"

(3). ln Direct Sequence Spread Spectrum, the most common form of

spread spectrum, each bit to be transmitted is converted into r

binary chips, where r is much g~eater than one and the original

signal bandwidth is expanded by the same factor of r in order

to accornmodate the higher transmission rate of the chips. Each

transmitter in such a network is usually assigned a distinct

5

spreading sequence and the receiver must then convert the stream

of .chips from each transmitter into a (lower data rate) stream of

bits by means of what is essentiall y a digital matched filter,

one matched to each distinct transmitter spreadi ng sequence. A

frequency hopping version of this procedure is also possible, but

frequency hopped spread spectrum seems better suited to military

rather than commercial applications.

packet

chips

Figure 4: spread spectrum -- From Bits to Chips

In the .1µ.0HA form of multiple access the bandwidth of the channel

is also required to be greater than the infonnation bandwidth

corresponding to the average data rate transmitted from any

single user in the channel. Each transmitter in such a multiple

access channel will transmit its data packets at the maximum data

rate of the channel. Because the channel bandwidth is hi gh

compared to the data rate from a single user, the duty cycle of

each user is low and the probability that two packets from

different users will overlap in the channel will be low. A

variety of protocols have been developed [4,5) to deal with the

case of packets which are lost due to contention in the channel.

6

The ratio of the maximum data rate which can be sustained in such

a channel to the maximum data rate when the channel is used by a

single user in a point-to-point mode is called the maximum

throughput of the ALOHA channel, and is equal to l/2e or o.184

(2). If the channel is modified so that each transmitter

synchronizes the beginning of its packet transmissions to fixed

slots then the maximum throughput of the channel is doubled to

l/e or 0.368 (6) .

t -

Figure 5: ALOHA Multiple Access

Note however that both the l/2e and the 1/e maxi.mum throughput

results compare the throughput of a low duty cycle (typically 10%

or less) ALOHA channel to that of a point- to-point channel

operated continuously. The continuous channel of course uses

considerabluy more power (typically 10 db. or more) than the

bursty ALOHA channel. For the case of the original packet radio

ALOHANET (2) whers the~ power is important such a throughput

comparison is appropriate. However in the case of most satellite

multi-access channels, and certainly in the case of VSAT

7

channels, where the limiting resource is the average power

transmitted in the do'W?!link, a different kind of comparison is

appropriate. In this case it makes more sense to compare the

data rate of the ALOHA channel with the data rate of a point-to­

point channel using the same average downlink power as the ALOHA

channel. And as might be expected, when proper allowances are

made for a 10 db. or more power mismatch, the ALOHA channel

throughput looks considerably more attractive. In s~ction 1.4 we

discuss the throughput os an ALOHA channel with an average power

rather than a peak power constraint.

1,3 Jlistory

Each of the two multiple access techniques described above was

originally developed for applications other than the satellite

channel. Spread Spectrum has its origins in a variety of

military applications [7) and much of the early work in spread

spectrum dealt with the transmission of continuous signals rather

than the packet transmissions of interest in most present day

systems. ALOHA channels were originally developed in the early

1970's for use in the ALOHANET, a Local Area Wireless Network

{LAWN) at the University of Hawaii [8]. The first demonstration

of the use of either of these techniques for satellite

communications however was in 1973 in the ALOHA SYSTEM'S PACNET

network, using the NASA ATS-1 satellite to link VSAT's in Hawaii,

Alaska, Japan and Australia .

8

In 1976 an unslotted ALOHA channel was implemented as the request

channel of the COMSAT MARISAT s ystem [9) in a commercial network

--- the first commercial application of this technology. The

first commercial use of spread spectrum in a satellite channel

was in 1981 with the introduction of the Equatorial

Co!lllllunications "C-100 micro earth station" for satellite data

distribution . The MARISAT ALOHA channel was a true multiple

access channel, providing many to one communications to thousands

of small ship stations in a three ocean system. The first

Equatorial Communications terminals were receive only units in

which only the interference rejection capabilities of spread

spectrum signals were utilized for data transmission. In 1984

Equatorial introduced a true multiple access satellite network

using the "C-200 Micro Earth Station". By 1985 several other

c~mpanies had entered the multiple access VSAT market in the US.

Ironically, by 1987, Equatorial which had first demonstrated the

commercial viability of the VSAT concept had run into troubl e in

its c-200 systems, and had been taken over by contel . By 1986

this market had matured so ·that several other companies had

entered the market and by the end of 1989 there were four major

supplier~ of this equipment in the USA -- TRIDOM/AT&T, Hughes

Network Systems/GM, GTE/Spacenet and Equatorial/ConTel. Each of

t~~~e systems ~as based on an ALOHA channel (either slotted or

unslotted) in some cases supplemented by TOMA for large file

transfers, although ConTel also offered its original Spread

Spectrum Multiple Access network as an option. By 1989 there

were about 10,000 multiple access commercial VSAT terminals in

9

operation not including the large number of ship stations using

ALOHA in the INMARSAT (formerly MARISAT) system.

1.4 satellite Multiple Access

Although both Spread Spectrum and ALOHA can provide a multiple

access capability in many different kinds of c9mmunicati9n

networks, there are a number of special characteristics of the

VSAT packet data multiple access channel which affect the

operation of the overall system. When spread spectrum is used t

distinguish among large numbers of intermittent packet

transmitters each using a different spreading sequence, the hub

receiving station must be provided with a digital matched filter

operating at the high speed chip rate for each of the possible

transmitters. In the Equatorial c-200 system these receivers ar

called "ear cards". And in a network with thousands of VSAT's

the logistics of installing, removing, maintaining and testing a

special piece of hardware at the hub station for each network

user is a difficult task.

The throughput of an ALOHA channel is ordinarily calculated on

the basis of comparing the average data rate of the burst ALOHA

channel to that of a full time dedicated point to point channel

between only two users. In the case of ground communications

where the average power radiated by the transmitter is not a

major constraint such a comparison is appropriate. But the

bursty ALOHA channel (see Figure 5) will ordinarily require 10%

or less of the average power required by the point to point

channel in such a comparison. In the case of a satellite channe

10

the average power transmitted from the satellite transponder is a

limited and expensive resource, so that a different kind of

comparison is appropriate.

For the satellite channel it is appropriate to begin the

comparison using Shannon's basic equation for the capacity of th~

bandlimited additive white Gaussian noise channel. In this case

R, the data rate of the channel in bits per second, is limited by

the channel capacity

(1)

where the logarithm is taken to the base 2, w is the channel . '

bandwidth, Sis the average signal power and N is the average

noise power. Shannon's equation assumes that the signal is

continuous, but if the time bandwidth product of the packets is

large enough we can apply Shannon's equation to the packets

during the tillles of packet transmission. And if the duty cycle

of packet transmissions is d { d << l) the data rate of the

multi-access ALOHA channel must be less than

C = dC 1 O

s = d w tog ( 1 + N) (2)

It is not difficult to find the corresponding capacity of a

point-to-point channel with the same average power as the ALOHA

channel . For a given duty cycle, d, the average power required

by the ALOHA channel is just dS, and the data rate of the

continuous channel must the r efore be less than

ll

(3)

Because of possible packet overlap equation (3) is valid for d<<l

and certain corrections must be made for the more general case.

These calculations are provided in [lO ; section VJ where the exact

derivation is provided for four separate cases the slotted and

unslotted ALOHA channel, operating with either a linear or a hard

limited transponder . In all cases the the basic results are the

same. As might be expected we always have

(4)

That is, the multi-access ALOHA channel will always have a lower

maximum throughput than the point-to-point channel with the same

average power. However, for all values of the duty cycle, the

ratio of the maximU111 ALOHA throughput compared to the capacity of

the average power limited channel exceeds the ratio of the

maximum ALOHA throughput compared to the capacity of the peak

power limited channel by a significant amount. And for the case

of a low signal-to-noise ratio in the receiving antenna (e.g.

VSAT's)

(5)

12

Thus we have the surprising result that f or the cas~ of small

s atellite earth stations operating with a low duty cycle, the

ALOHA capacity approaches the Shannon capacity, and it is not

possible to find an access method with a higher data throughput

[10; section VJ.

For the kind of ALOHA operation described in the previ ous

paragraph, in order to approach the Shannon channel capacity it

is necessary to operate at a low duty cycle, and therefore for

interesting values of average power, the peak power of the VSAT

trans~itter during data bursts can be quite high. And this

characteristic of the low duty cycle ALOHA channel could limit

the application of this mode of operation in satellite networks

composed of large numbers of VSAT's . Since the signal

detectability depends upon the energy per bit and not the average

signal power it makes sense to spread the transmitted packets in

time in order to decrease the peak power requirements of the

earth station transmitters while keeping the transmitted energy

per bit constant. Note that this strategy can be used in the

case of a burst ALOHA channel without decreasing the average data

rate of the channel . In section 3, we show how it i s possible to

spread the packets of a low duty cycle ALOHA channel in time,

without increasing the probability of packet overlap out of the

channel receiver. But first we use section 2 to i ntroduce a

notation which will be helpful in the remainder of this paper.

13

2, NO'rATION

2,1 Packets &lid Matched Filters

Consider the case of packet transmission where each packet consists

of exactly n bits. Let the value of these bits be dj, where the

data ·sequence dj is given by

~-±1 j-0,1 ... n·l (6)

In this paper we s hall be concerned with binary phase-shift keyed

(BPSK) channel s for transmission of packets and with the

representation of these packets in a form which emphasizes their

correspondence to conventional direct sequence spread spectrum

channe l s [11). Let p(t) be the bit pulse waveform used in the

channel so that a single packet, D(t), consisting of n bit

transmissions at the times 0,1, • • • n-1

n·l

O(c) • L dJp(C•j) j-0

can be represented as

(7)

A useful device which allows us to separate the analog effects of

pulse shape from the digital effects of the data sequence is to

represent Q(t) as the co~volution of the pulse p(t) wi th a sequence

of impulses multiplied by the data sequence values dj. Then

•·l D(t) • L d,&(t·j) * p(c)

J•O (8)

Note that D(t) can be thought of as the output of a time invariant

linear operator with impulse response p(t), when the input is

14

n•l

d(e> - I dj6<t·j> (9) J••

d(t) D(t) I p(t) I Figure 6: The Packet as the output of a Linear Filter

The reception of a sequence of packets occurring at random times

in a multiple access channel is usually broken down into two

stages a packet detection and synchronization stage (to

detect the presence of the packet and to synchronize) and a

signal detection stage (to demodulate the signal and to determine

the values of the information bearing elements. In a VSAT

network the synchronization problem is made considerably simpler

by the fact that the hub station can provide information feedback

to the VSAT terminals in order to help them synchronize the phase

of their packet transmissions. We therefore need only consider

the signal detection part of the receiver. We assume the packet

O(t) is transmitted in a channel wi th additive white Gaussian

noise (AWGN), n(t), so that the received signal a(t) is

a(t) • D(t) + n(t) (10)

15

Under a wide variety of assumptions, a key element of the packet

receiver [12,13] is the matched filter matched to the pulse

shape, p(t), Ignoring questions of time delays in the receiver,

we can represent the impulse response function of the matched

filter as p(-t), and the output of the matched filter as b{t)

[ 14, 15] •

a{t) • D(t) + n(t)

I b(t) p(-t) •~~~-

Figure 7: Input and output of the Matched Filter

Then the output of the matched filter is given by

li(t) • a(t) • p(·t)

- D(t) • p(•t) + n(c) • p(•t)

n•l

•}: dJ6(t·j) • p(c) • p(-c) + n(c) • p( -c) J•Q

Define the correlation of the bit waveform as

p(C) • p(C) • p( • C)

• • f p(X)p(C•X) d.~ ...

Then,

16

(11)

(12)

•·1 b(t) • 1 dj6(t•j) • p(t) + n(t) • p(·t)

J•O

n•I (13) - r djp(t•j) + n,(t) J•O

where we define

n1(t) • n(t) * p(·t) (14)

Many decision rules to obtain the data sequence, dk, from the

received signal rely on sample values of b(t) taken at t equal to

the bit transmission times k = O,l,2 •••

o•I

b(k> - E djp<k·j> + n 1<k> J•O

• <1i,p(O) + I(k) + n1(k) (15)

where n•l

I ( k) - L djp(k·j) j-0

J•k ( 16)

is the inter-symbol interference for the k'th symbol in the packet.

The energy per bit is

• 2

~ - f p (t) dt ...

Then, if N0 is the power spectral density of the noise, E!)IN0

provides a measure of the quality of the signal detection phase

of the receiver [16).

17

(17)

2.2 Spreading Sequences

In order to achieve the time spreading of the packets discussed

in section 1.4 we use binary sequences with low autocorrelation

properties, such as Barker sequences [17,18]. Define a binary

sequence of length r as

j • O,l ... r-1 ( 18)

and a time invariant linear operator (spreader) corresponding to

the sequence sj with an impulse response function

.c-1 .

s(t) • L sJ6(t·j) J•O

Define the correlation of s(t) as

where

a(t) • s(t) • s(-t)

• - J s(x~ s(x+t) dx -

r-1

• l "J6(t·j) J•l·r

r·l

"J • sJ•sJ - l sk s,. J k•O

18

(19)

(20)

(21)

In the case of Barker sequences,

(22) for j•O

and

for J"'O

(23)

Barker sequences are known only for the cases of r-2,3,4,5,7,ll

and 13, but binary sequences of other lengths with good values of

for use in Spread ALOHA channels as well as efficient methods of

searching for such sequences have been obtained by Li (18).

3, SPREAD ALOHA

3,1 Spreading the Packets

As explained in section 1.4, the low duty cycle ALOHA channel shown

in Figure 1.5 can be used in VSAT networks with one major theoretica

advantage (it can achieve the Shannon bound) and one major practical

disadvantage (it may r equire a high value of peak power out of the

small aperture earth station). In this section we modify the

signals in the low duty cycle ALOHA channel so that the disadvantage

of high peak power is eliminated while at the same time EbJ'N0 the

measure of signal quality defined in section 2.l remain unchanged.

We call this modification "Spread ALOHA", and our results show that

the performance of Spread ALOHA is the same as that of low duty

cycle ALOHA while the performance of the latter is the best possible

for a given average power and a given channel bandwidth.

19

The modification we use is a natural one --- we take the packets

shown in Figure 5 and spread them in time. such time spreading can

be done in a number of ways, but the method we describe in section

3.2 has the property that the spreading leaves Et)IN0 unchanged and

at the same time the spreading does not affect the probability of

packet overlap at the output of the detector. If the satellite

channel is operated in a linear mode, the increased channel overlap

will not affect the operation of the ALOHA channel since the optimum

detector consists of a matched filter which serves to despread the

spread packets .

3.2 Bit Spread ALOHA

our objective then is simply to spread the in time, thus effectively

lowering the transmit power requirements of the VSAT terminals,

while at the same ' time keeping the transmitted energy per bit

constant, and also by means of the despreading operation at the

network hub, keeping the effective overlap silhouette of the

transmitted packets constant. Note the distinction made here

between the overlap of packets in the physical channel which is

greatly increased by Spread ALOHA, and the overlap of packets at the

output of the detector which is not.

For Bit Spread ALOHA packets we start with equation (7) and delay

the transmission of each bit by r units of time relative to the

previous bit. That is, let the transmitted packet be

20

n•l

D,(t) - 1 dJp(t-rj) J•O

and, following the development of equations (8) and (9)

n· l

· D,(t) - I dJ6(t -rj) * p(t) J-0

(24)

(25)

As before we can represent Or(t) as the output of a time invariant

linear operator with impulse response p(t) , when the input is

•·1 d,(t) - 1 dJ6(t-rj)

J-0

p(t) I

(26)

D,(t)

Figure a: The stretched Packet as the output of a Linear Filter

We refer to the packet Or(t) as the stretched packet. And we

produce the spread packet we want by passing the stretched packet

through a spreader, a time invariant linear filter with impulse

response equal to r-l/2 s(t), as given in equation (19).

p(l:) I D,(t)

I D

1(t)

,:·112 s(t: •-----

Figure 9: The Bit Spread Packet as the output of Two Linear Filters

21

The process of spreading an ALOHA packet for a simplified packet is

illustrated i n-Figure 10. In order to clarify the process the bits

and chips are represented without the pulse shaping filter p(t).

The first line of Figure lO depicts a packet composed of 6 bits --­

llOlOl . The stretched packet of equations (24) and (26) is shown on

the second line. The third line illustrates the spread packet for

this simple example, where the spreader is taken as the length 7

binary sequence of line 4 --- 1110010 . Of course more realistic

values of both the packet length and the length of the spreading

sequence would be in the range from lOO to 10,000.

II I I Packet : 118181

I I

_.j._ __ -'---~----11--------'--- Stretched PacJcet I

' I I I t I

II! I Ill I ; II IIIJ I : IJ llfl J Spread Packet

I I I I I I I I I I I I I I I I I I I I

l I I I Spre.acl i 119 Se<{uenc.e: 1110818

II I Figure 10: EXa.mple of a spread ALOHA Packet

The form of the spread packet is now easily calculated as

D,(t) - r·112 d,( t) * s( t) * p(t)

rn-1

- t cjscc-J> * p<c> j-0

rn- 1

- t cjp<c·J> j-0

22

(27)

where

C - r·l/2 d s. j O ~

(28)

and «: is the largest integer less than or equal to j/r and /3 is equal to j modulo r

a - LJ/rJ ,8 • j mod r

(29a)

(29b)

The binary random variables, cj, constitute the "chips" of the

Spread ALOHA signal; more precisely, cj is the /3 'th chip of the

"<' 'th data bit of the spread signal. Since each bit of the original

packet is converted to r chips of the spread packet by the

spreader, each chip is multiplied by r-1/ 2 in order to maintain

the same energy per bit before and after spreading. Thus the peak

power requirement of the transmitter is decreased by a factor of r .

Time spreading provides a mechanism for reducing the average power

of the transmitter to a level consistent with a network of VSAT

terminals while maintai ning a constant value for the energy per bit.

Note that in contrast to conventional spread spectrum for continuous

signals, each packet in the Spread ALOHA channel is spread by the

same binary sequence, s j . Separation of packets from different

users is accomplished by means of the ALOHA contention protocol

rather than by the cross-correlation of different spreading

sequences. And in contrast to conventional operation of an ALOHA

channel the separation is .not limited by the ove rlap of the

transmitted packets in the channel, but by the overlap of these

pulses at the output of a matched filter .

23

From Figure 9 we see that the problem of detection of the bits of

the spread pa~ket in the presence of AWGN has been reduced to that

of the unspread packet of Figure 2. We need only replace p(t) in

Figure 6 by the convolution of p(t) and s(t) in Figure 9. Then

f or the spread case the spread matched filter is shown in Figure ll.

b(t) a(t) • 01 (t) + n(t) I I I -------:__p•(•-•t•)-• r"

112

s(-t: •-----

Figure 11: Input and output of the Bit spread Matched Filter

Now following the development used in equation (ll} we have

b(t) - r"112[a(t) • p(•t) • s(·t)]

- r·112 [D (c) • p(·t) • s(·t)] + r"112 [n(t) • p(·t) • s( -c:)] • - r"1{d (t) • s(t) • s( •C:) • p(c) • p( ·t) ] + r·112[n(t) • p( ·t) • s(·t)]

<

where we define

llz(C) - r"112(n(C) • p(•t) • S(•C))

(30)

(31)

Using (20) and (26), we can expand b(t), the output of the matched

filter in equation (30)

24

n·l r--1

b(C) - r"1 r dJ"(C•rj) * r c,J6(t•j) * p(C) + tiz(t) J•O J•l•r (32)

If we define d_1 a dn = 0 and <5"" • o , then -r

nt•l

b(c) - I: ~,6(t•j) • p(t) + nz(t ) (33) J•l·t

nr• l - r ejp(c-j) + nz(t) j•l·t

where

(34)

and OC. is the largest integer less than or equal to j/r and /3 i s equal to j modulo r.

a - l)/rj

{J • j mod r

(3!5a)

(35b)

The output of the matched filter is s ampled at the stretched bit

transmission times t•kr, where k~o,1,2 ••• n-1. At these sample

points /3: 0 and a;= r , so that ekr = dk, and we get

25

where

nt·l

b(kr) - L eJp(kr·j) + n2(kr)

J•l·~

• di.P( O) + J(kr) + n2(kr )

.... 1

J (kr) - L eJp(kr- j) J•l·~ J,ikz

is the inter-symbol interference for the k'th symbol in the

packet.

3.3 I>iscussion

(3')

(37)

Note the similarity of the interference term of equation (37) to

that of equation (16) • . The sample values of the data pulse

correlation, ('(t), caus~ the interference. In this case the

interference might properly be termed "inter-chi p interference"

rather than inter- symbol interference since the stretched packet

has separated each bit by r units of time and, for reasonably

shaped pulses, p(t) , this will eliminate the possibility of

inter-symbol interference. And even in those cases where the

sample values of . j'(t) are not negligible note that the

coefficients ej as given in equation (34) decrease as r-1 • This

means that ·the interference caused by adjacent chips in Bit

Spread ALOHA will be considerably less than that caused by

adjacent bits in unspread packets.

26

Finally we note that packet samples utilized by a matched filter

detector of Data Spread ALOHA packets are taken at intervals of r

units of time. In the case of the AWGN channel assumed, packets

which overlap in the channel can still be detected correctly as

long as all samples of each of the two packets are separated by a

single chip interval, taken as a unit of time . Thus the total

period of vulnerability in this unslotted Spread ALOHA channel is

twice the packet length, just as in a conventional unslotted

ALOHA channel. The only difference in this case is that the

period of vulnerability is broken up into 2n-l small

subintervals. In contrast to conventional ALOHA channels two

packets which overlap in the channel do not necessarily . r esult in

lost packets due to collision. The spreading and despreading

operations effectively separate most of the channel collisions in

Spread ALOHA signals so that the only overlap which· is of concern is

the overlap at the output of the matched filter detector .

The spreading technique we have described is not the only

possible method of achieving the practical advantages o! Spread

ALOHA. In (19] a similar dual spreading procedure, called Chip

Spread ALOHA is analyzed and an algebraic coding formulation of

Spread ALOHA is provided.

3.4 conclusions

The arguments o! equations (1) to (5} show that the usual analysis

of ALOHA throughput for peak power limited channels is not

appropriate for the case of average power limited VSAT channels.

The well known l/2e • 0.184 and 1/e = 0.368 limits compare the

27

performance of a multiple access ALOHA channel to that of a point­

to-point channel with significantly greater average power in the

downlink. Simply put, some of the results originally published by

this author are too often used as the right answer to the wrong

question.

When the constraint of identical average power in the satellite

downlink is taken into account, the maximum value of the throughput

of conventional slotted or unslotted ALOHA channels will always be

higher than the l/2e or 1/e limits, and in the case of VSAT networks

typical of those available today, significantly higher. The

greatest improvement will occur for the case of low signal-to-noise

ratios in the VSAT receiver and low duty cycle transmission --­

precisely those conditions of greatest interest in VSAT networks

(101 section VJ.

For VSAT networks with an average power limitation in the downlink,

as the bandwidth used for the multi-access channel is increased the

signal-to-noise ratio in the VSAT receivers and the duty cycle

defined in terms of the multi-access bit rate (not the chip rate)

both decrease. In the limit the maximum value of the throughput

approaches the Shannon bound and it is not possible to find an

access technique with a higher throughput.

In order to achieve reasonable values of signal energy per bit using

conventional ALOHA operating at a low duty cycle in the high

bandwidth channel it might be necessary to require unreasonably high

values of transmitter power from the VSAT terminals. The solution

to this problem is to spread the VSAT packets in time to obtain a

28

low power Spread ALOHA signal. In this case the Spread ALOHA signal

becomes identical in form to a COMA spread spectrum signal, and in

the limit both types of tranSl!lission achieve the Shannon bound.

Separation of the signals from different transmitters at the huh

station however can utilize the ordinary signal separation mechanism

of the ALOHA channel rather than the chip code separation of the

CDMA channel.

29

REFERENCES

l. Edwin B. Parker, "Micro Earth Stations as Personal computer Accessories", Proceedings of the IEEE, vol. 72, no. 11, November 1984, pp. 1526-1531.

2. N. Abramson, "Packet switching with Satellites," AFIPS Conference Proceedings of the National Computer Conference , New York. Vol. 42, pp 695-702. June 1973

3. Andrew J . Viterbi, "Spread Spectrum communications and Real ities," IEEE Communications Magazine, Vol . 17, 1979, pp. 11-18.

Myths no. 3, May

4 . Dimitri Bertsekas and Robert Gallage r, "Data Networks", Prentice­Hall, Inc., Englewood Cliffs, New J e r sey

5. Simons. Lam, "Satellite Multiaccess Schemes for Data Traffic", Proceedings of the 1977 International Conference on Communications, Vol. III, pp 37.1.19-37.1.24, June 12-15, 1977.

6 . Roberts, Lawrence G., "ALOHA Packet System With and Without Slots and Capture", Computer Communication Review, Vol. 5, No. 2; April 1975; pp. 28-42

7. Robert A. Scholtz, "The Spread Spectrum Concept," IEEE Transactic Communications, Vol. COM-25, no. 8, August 1977, pp. 748-755.

8. Norman Abramson, "Development of the ALOHANET", IEEE Transactions on Information Theory, Vol. IT-31, No. 2, March 1985

9. D. W. Lipke, D.W. Swearingen, J.F. Parker, E.E. Steinbr echer, T.O. Calvit and H. Dodel, "MARISAT - a Maritime Satellite Communication System", COMSAT Technical Review, Vol. 7, No·. 2, Fall 1977.

10. N. Abramson, "The Throughput of Packet Broadcasting Channels," IEEE Transactions on Communications, Vol. COM-25, no. 1, January 1977, pp. 117-128.

11. Raymond L. Pickholtz, Donald L. Schilling and Laurence B. Milstein, "Theory of Spread-Spectrum Communications --- A Tutorial," IEEE Transactions on Communications, Vol. COM-30, No. 5, May 1982, pp. 855-884

12. Michael B. Pursley, Dilip V. Sarawate and Wayne E. Stark, "Error Probabili ty for Direct-Sequence Spread-Spectrum Multiple­Access Communications ---Part I: Opper and Lower Bounds ," IEEE Transactions on Communications, Vol. COM-30, N. 5, May 1982, pp. 975-984.

13. Norman F. Krasner, "Optimal Detection of Digitally Modulated Signal," IEEE Transactions on Communications, Vol. COM-JO, no. 5, May 1982, pp. 885-895.

ref-1

14, George L. Turin, "An Introduction to Matched Filters," IRE Transactions on Information Theory, Vol. IT-6, June 1960, pp. 311-329.

15. George L. Turin, "An Introduction to Digital Matched Filters,u Proceedings of the IEEE, Vol. 64, No. 7, July 1976, pp. 1092-1111.

16. George R. Cooper and Clare D. McGillem, Modern Communications and Spread Spectrum, McGraw-Hill Book Company, New York, 1986.

17. R.H. Barker, "Group Synchronizing of Bi nary Digital Syste111S," in Communication Theory, edited by w. Jackson, Academic Press, 1953, pp. 273-287.

18. Ping-Fai Li, "Binary Sequences with Low Autocorrelation," ALOHA SYSTEM Technical Report B86-l, University of Hawaii, Honolulu, May 1986.

19. Norman Abramson, "Spread ALOHA for VSAT's", ALOHA System Technical Report B86-4, University of Hawaii, Honolulu. June, 1986.

ref-2